1. Field of the Invention
The present invention relates to a thin-film transistor array and an image display device in which the thin-film transistor array is used.
2. Description of the Related Art
With an outstanding progress of an information technology, currently information is frequently transmitted and received with a notebook personal computer or a personal digital assistance. There is well known the fact that a ubiquitous society in which the information can be transmitted and received anywhere comes in the near future. In the ubiquitous society, there is a demand for a lighter, lower-profile information terminal.
Although a current mainstream of a semiconductor material is a silicon system (Si system), a study on a transistor in which an organic semiconductor or an oxide semiconductor is used becomes active from the viewpoint of a flexible information terminal, weight reduction, cost reduction, and the like. Generally, in the case that the organic semiconductor is used, advantages such as enlargement of an area, application of a printing method, and use of a plastic substrate can be cited because a process can be performed in a liquid state (see Non-Patent Literature 1). Some kinds of oxide semiconductors can be deposited at low temperature and exhibit high carrier mobility. For example, there is proposed a field effect transistor that is made of an amorphous In—Ga—Zn—O material as the oxide semiconductor (see Non-Patent Literature 2). In Non-Patent Literature 2, the material of the amorphous oxide semiconductor is used as a semiconductor active layer, whereby a transparent field effect transistor having excellent carrier mobility of about 10 cm2/Vs is successfully fabricated on a PET substrate at room temperature.
However, the carrier mobility of the organic semiconductor is lower than that of amorphous silicon. For example, in the case that a thin-film transistor array that drives a display is made of the organic semiconductor, it is necessary to form a relatively large transistor in order to ensure a current value necessary for drive. Therefore, unfortunately a ratio of the transistor to one pixel increases to decrease an aperture ratio (an area ratio of a display portion to a pixel unit). For example, in an example of a conventional thin-film transistor illustrated in FIGS. 26 and 27, a source electrode 27 extending from a source line 28 and a drain electrode 26 connected to a pixel electrode 25 are formed into a comb shape, and a semiconductor layer 12 is formed between the source electrode 27 and the drain electrode 26. A region of the pixel electrode 25 is narrowed by a thin-film transistor forming region including regions where the source electrode 27 and the drain electrode 26 are formed, and therefore the area ratio (aperture ratio) of the pixel electrode 25 is decreased.
Therefore, there is a method, in which an interlayer insulating film 15 is formed above the thin-film transistor and an upper pixel electrode 29 is formed on the interlayer insulating film 15, thereby increasing the aperture ratio as illustrated in FIGS. 28 and 29.
In the case that the oxide semiconductor is used, although a size of the transistor can be reduced because of the high carrier mobility, the above method is also adopted in order to further increase the aperture ratio.    Non-Patent Literature 1: Science Vol. 265, 1684 (1994)    Non-Patent Literature 2: K. Nomura et al, Nature 432, 488 (2004)
However, in the case that the aperture ratio is improved by the above method, the cost increases with increasing the number of processes.
A screen printing method or a photolithographic method is used as a process of forming the upper pixel electrode 29. However, when the screen printing method or the photolithographic method is used after the semiconductor layer is formed, stabilities of a threshold voltage and the carrier mobility of the thin-film transistor are degraded.